1. Field of the Invention
The present invention relates to a digital color image signal processing apparatus.
2. Brief Description of the Related Art
One exmaple of conventional color image signal processing circuts for a digital color copier is shown in FIG. 8. As shown there, three digital color luminance signals R (red), G (Green), B (Blue) produced by a CCD (charge coupled device) 301 are converted into three digital color density signals D.sub.R, D.sub.G, D.sub.B by a density conversion circuit 302. Then the three digital color density signals D.sub.R, D.sub.G, D.sub.B are input to a black processing circuit 303 which comprises a black component generation circuit for generating a black component BK, and an UCR (Under Color Removal) for respectively removing the black component from the three digital color density signals D.sub.R, D.sub.G, D.sub.B to obtain digital signals D.sub.R, D.sub.G, D.sub.B exlcuding the black component. A masking circuit 304 is provided for correcting color signals or removing asymmetrical components, and outputs C (cyan), M (magenta), Y (yellow) signals of the masking circuit 304 and the black component BK from the black processing circuit 303 are supplied to a color printer 305. The color printer 305 reproduces a color image in accordance with supplied signals C, M, Y, BK.
FIG. 9 shows a conventional block diagram in greater detail of the a main part of color image signal processing circuits shown in FIG. 8. As can be seen there, the three digital color luminance signals R, G, B from the CCD 301 (FIG. 8) are respectively input to three look-up tables (LUT) 204, 205, 206 which comprise random access memories (RAM). The LUTs 204, 205, 206 convert the input color luminance signals R, G, B into the three digital color density signals D.sub.R, D.sub.G, D.sub.B. FIG. 10 shows a characteristic for conversion of the luminance signals R, G, B into the density signals D.sub.R, D.sub.G, D.sub.B. In this case, each of the luminance signals R, G, B is an 8-bit signal, namely each luminance signal has 256 steps of halftone gradation. Each characteristic of the LUTs 204, 205, 206 is able to be expressed by the following relations: ##EQU1## where F.sub.R, R.sub.G, F.sub.B respectively are functions of three characteristics of the LUTs 204, 205, 206.
A minimum value determining circuit 207 detects and outputs a minimum value from among the density signals D.sub.R, D.sub.G, D.sub.B and a multiplier 208 multiplies the minimum of the density signals, MIN (D.sub.R, D.sub.G, D.sub.B), by a constant K (O&lt;K.ltoreq.1) in order to obtain the black component signal BK. That is: EQU BK=K.times.MIN (D.sub.R, D.sub.G, D.sub.b) (2)
Three subtractors 209, 210, 211 are also provided for respectively subtracting the black component signal BK frome ach of the three density signals D.sub.R, D.sub.G, D.sub.B in accordance with he following relation: ##EQU2## A masking processing circuit 212 comprises multipliers and adders, and performs the operation of the following relation. ##EQU3## where {A.sub.IJ } is a linear 3.times.3 matrix, and C, M and Y respectively represent the resulting cyan signal, magenta signal and yellow signal.
Then signals C, M, Y and BK are supplied to the digital color ptiner 213 which i capable of reproducing color gradation.
In the above mentioned conventional processing circuit, however, output color signals C, M, Y are obtained from input color signals R, G, B, by being processed through a series of adders, subtractors and multipliers. Therefore, the signals C, M, Y may have digital operational errors. In order to reduce the digital operational errors, each processing circuit should increase the number of bits used in the processing. However, this causes the size of the processing circuit nessarily to become large.
As shown in FIG. 10 and FIG. 11, the characteristic of conversion is very similar to a logarithmic function. Thus, if a simple LUT is provided as a convertor for converting from a luminance signal to a density signal, gradation loss occurs especially in the low density level area (highlight area).
In order to prevent the gradation loss in the low density level are, the number of output bits of the LUT is set greater than a number of input bits as shown in FIG. 12 and FIG. 13. FIG. 12 shows a characteristic of a LUT 130 in FIG. 13. However, in FIG. 9, if the number of bits of each density signal D.sub.R, D.sub.G, D.sub.B is increased, other processing circuits in FIG. 9 must be specified appropriately to properly operate on such additional bits. Therefore, the scale of each processing circuit also becomes very large.